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Lesson 10: Interfacing and Interrupts

From "AMERICAN LABORATORY"
by David G. Larsen, Peter R. Rony, and Jonathan A. Titus

Interfacing a Digital Multimeter
THIS MONTH, we shall discuss the interfacing of an 8080based microcomputer to a very versatile laboratory instrument, the Keithley model 160B digital multimeter with model l602B digital output. After a year's use, we have found this multimeter to be an excellent example of what manufacturers can do to facilitate the interfacing of their instruments.

The model l60B is a general-purpose 3(1/2)-digit multimeter that can function as a de voltmeter, dc ammeter, or ohmmeter. A total of 26 different ranges exist for the multimeter in its three modes of operation. The lowest range scales provide maximum readings of 1.999 mv, 19.99 nA, and 1.999 Q. The 1.999 mv scale has an accuracy of ±0.1% of reading ±l digit. Thus, a display reading of 1.000 mv will have an uncertainty of ±0.002 mv, or 2 J.1v. The highest possible readings associated with the three different modes of operation are 1200 v, 1999 mA, and 1999 MQ, with the megohm reading being accurate to only ±30%. This multimeter can be viewed as the digital complement of the ubiquitous multirange chart-paper recorder. The multimeter is basically a sophisticated analog-to-digital converter (ADC) that can handle most laboratory requirements for digital data acquisition provided that the data acquisition rate is no greater than one data point per second. * Switching between the 26 different ranges is performed manually. We would expect that, in the future, such switching will be performed by a built-in microprocessor operating under the control of an external computer.
Read More: Adobe Acrobat PDF FileInterfacing A Digital Multimeter.pdf [45.2KB]

Microcomputer Interrupts
THIS month's column is the first of several that will focus on the concept of an interrupt. When used in the context of a computer, an interrupt can be defined as the suspension of normal program execu tion in order to handle a sudden request for service, i.e., assistance, by the computer. At the completion of interrupt service, the computer resumes the interrupted program from the point where it was interrupted;' This specific use of interrupt is consistent with the general meaning of the term: to stop a process in such a way that it can be resumed.

A given computer will typically communicate with avariety of external I/O "devices." If it is a minicomputer, it may communicate with a Teletype or alphanumeric keyboard, a CRT display, a printer, a floppy disk, and perhaps one or more laboratory instruments. If it is a microcomputer, it may communicate with smaller devices-motors, solid-state relays, push-button switches, display lights, etc.-within a larger machine or instrument. When used as a replacement for discrete logic devices in a complex digital circuit, a microcomputer may communicate with other TTL-integrated circuit chips such as latches, flip-flops, and three-state buffers.

When communicating with external I/O devices,2 microcomputers can operate in two general modes, polled and interrupt. Polling is the periodic interrogation of each I/O device sharing a communications link to a microcomputer in order to determine if it requires servicing. A microcomputer sends a poll that has the effect of asking the selected device, "Do you have anything to transmit?," "Are you ready to receive data? ," and similar questions. When a microcomputer services a polled device, it simply exchanges digital information with the device in a manner that is prescribed by software in a subprogram or subroutine called a software driver.
Read More: Adobe Acrobat PDF FileMicrocomputer Iterrupts.pdf [45.2KB]

The Vectored Interrupt
THIS MONTH, we continue the discussion of computer interrupts, with emphasis upon vector interrupt hardware and software associated with the 8080A microprocessor chip. The three signals used in vector interrupt circuits include INT (input pin 14 on the 8080A chip), INTE (output pin 16), and INTA, not available on the 8080A chip but derived externally with additional logic.

A positive clock pulse from an interrupting device supplies a logic 1 state at the INT, or interrupt request, input that generates an interrupt request, which the CPU recognizes either at the end of the current instruction being executed or while the CPU is in the halt state. The INTE, or interrupt enable, output pin indicates the logic state of the interrupt enable flipflop present within the 8080A chip. This internal flip-flop can be set (enabled) or cleared (disabled) with the aid of 8080A microcomputer instructions :

363 DI Disable interrupt flip-flop
373 EI Enable interrupt flip-flop

When cleared, the interrupt enable flip-flop inhibits interrupts from being accepted by the CPU. The flip-flop is automatically cleared when an interrupt is accepted; it is also cleared by the RESET input signal applied at pin 12 of the 8080A chip.
Read More: Adobe Acrobat PDF FileThe Vectored Iterrupt.pdf [45.2KB]

Interfacing a 10-bit DAC
IN THIS COLUMN we shall discuss how to interface an Analog Devices AD7522 monolithic CMOS lO-bit multiplying digital-to-analog converter (DAC), a 28-pin chip which is a recent example of a new generation of inexpensive DACs that can be interfaced directly to 8-bit microcomputers.

A digital-to-analog converter is an electronic device that converts digital signals into analog signals. A typical converter consists of an arrangement of weighted resistors, each controlled by a single bit of input data that develops varying output analog voltages or currents in accordance with the digital input code.
Read More: Adobe Acrobat PDF FileInterfacing A 10 Bit DAC.pdf [165KB]

Interfacing Analog-to-Digital Converters
AN analog-to-digital converter is an electronic device that converts analog signals to digital signals. Typical commercial converters are based upon the principles of successive approximation, dual-slope integration, staircase-ramp conversion, or voltage-to-frequency conversion. I The most common use for an ADC is to convert the output from an analog transducer or analog instrument into digital form suitable for direct observation on a digital display or as input into a computer. All digital panel meters and digital rriultimeters contain built-in analog- to-digital converters. Modern ADCs provide standard TTL outputs which may be coded in binary, binary coded decimal (BCD), or perhaps other less frequently used codes.
Read More: Adobe Acrobat PDF FileInterfacing Digital To Analog Converters.pdf [69.7KB]


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